bitcoin: create new wrapper type bitcoin_txid, log backward endianness.

It's just a sha256_double, but importantly when we convert it to a
string (in type_to_string, which is used in logging) we use
bitcoin_txid_to_hex() so it's reversed as people expect.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/generate-wire.py

@@ -19,6 +19,7 @@ type2size = {
     'struct pubkey': 33,
     'struct sha256': 32,
     'struct sha256_double': 32,
+    'struct bitcoin_txid': 32,
     'u64': 8,
     'u32': 4,
     'u16': 2,
@@ -78,6 +79,7 @@ partialtypemap = {
     'features': FieldType('u8'),
     'channel_id': FieldType('struct channel_id'),
     'chain_hash': FieldType('struct sha256_double'),
+    'funding_txid': FieldType('struct bitcoin_txid'),
     'pad': FieldType('pad'),
 }